If you hike in the Northeast, Upper Midwest, Rocky Mountains, or Sierra Nevada, chances are your favorite trails were once buried under ice sheets thousands of feet thick. And while the last great ice sheets receded from North America 12,000 years ago, evidence of their impact is still present today. “Without glaciation, the Sierra Nevada, Rocky Mountains, and New England ranges would look more like the Great Smoky Mountains,” says Dr. William Locke, a glaciologist at Montana State University. That’s because the grinding ice sheets in those places eroded the soil that had formed over the preceding eons, exposing the bedrock features we see today from Mt. Washington to Longs Peak. Here’s how to spot classic glacial leftovers found in four national parks–and in many other backcountry spots carved by ice.

>>Andrews Tarn Although glaciers first appeared in RMNP up to 500,000 years ago, most of the features recognizable today were carved during the Pinedale glaciation, 35,000-10,000 years ago. The Andrews Glacier, a remnant of the Arapahoe Peak glaciation that lasted until 1850, feeds Andrews Tarn from the cirque above in the Loch Vale area of the park. Reach it via a strenuous 9.2-mile roundtrip from the Glacier Gorge trailhead that gains 2,150 feet of elevation.

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